The relationship between aerosol number concentration and volume, or mass, concentration is a key linkage for the impact of aerosols on cloud albedo and thus climate forcing. Furthermore, this relationship is important in interpreting aerosol remote retrievals by satellites. We report here aircraft data, primarily from the TARFOX (Tropospheric Aerosol Radiative Forcing Experiment) study, which suggest a general linear relationship between aerosol number and mass concentration in the effective cloud condensation nucleus (CCN) size range (diameter >0.1 μm) for the locations examined. Limited data on the relationship between aerosol number concentration and chemical composition are also presented, which show a preferential dependence of effective CCN number concentration on sulfate mass. However, an implicit dependence on organic mass is also suggested. ¿ 1998 American Geophysical Union